1. Field of the Invention
The present invention generally relates to a transmission for self-propelled working vehicles. More particularly, the invention relates to a transmission for a self-propelled working vehicle in which the transmission is likely to experience large variations in the load thereon.
2. Description of Relevant Art
There are known self-propelled working vehicles of the type which includes an engine, a drive wheel, and a friction disc type transmission interposed therebetween to transmit drive power from the engine to the drive wheel, the transmission having a pair of friction plates arranged perpendicular to each other, and which further includes a working implement adapted to be driven by the engine, such as proposed in the U.S. Pat. No. 3,580,351 by I. J. Mollen, patented as of May 25, 1971.
In the U.S. Patent by Mollen, the working implement is given in the form of the combination of a snow-gathering auger and a snow blower. In other words, the U.S. Patent by Mollen has disclosed a self-propelled working vehicle in the form of a snow-removing tractor.
Although, when such snow-removing tractor is moving on the snow of which accumulation may be uneven depending such as on the ground surface and snow-fall conditions, the load that the transmission experiences may vary in accordance therewith, the transmission of friction disc type is able to effectively absorb such load variation, thus permitting the tractor to travel smoothly.
However, such snow-removing tractor is adapted to proceed performing a snow-removal work and thus has a travelling speed thereof preset to be remarkably low as of a vehicle, that is, as low as a walking speed of man, so that also the travelling to and from the working place is very slow.
To overcome such shortcoming, the transmission may first be so modified as to have at the drive side thereof a friction disc enlarged in the diametrical size and at the driven side thereof another friction disc correspondingly enlarged of the travel range. Such modification, however, is to unavoidably extend the dimensions of the transmission itself, thus scaling up the snow-removing tractor. In this respect, when taking into consideration the load variation attendant the travelling on the snow, it may be most favorable to employ a transmission of the friction disc type which is able to be adapted with a relatively simple construction to cope with such load variation. However, dimensional parameters of conventional transmissions of such type are determined to be best effective at a very low travelling speed, which implies that no modifications such as by the enlargement of drive disc diameter may bring into existence more than the lowerment in the durability of such transmission. This is because expanding the travel speed range of snow-removing tractor in such a manner as described above is to expand the reduction gear ratio of transmission, which means subjecting a friction disc thereof to larger torques.
In other words, it may be considerably difficult to expand the travel speed range of such snow-removing tractor without marring the inherent function of a friction disc type transmission and without enlarging the size thereof.
On the other hand, as a second possible modification, also the interposition of a sub-transmission between the friction disc type transmission as a main transmission and the drive wheel may well serve to expand the travel speed range of the snow-removing tractor. Such interposition, however, is to again give rise to an expansion of the reduction gear ratio of power transmission system like the case of the foregoing modification, thus causing the main transmission to be directly subjected to an increasing torque in reverse proportion to the gear ratio expansion, which is unfavorable to the durability of the main transmission.
In this respect also, it may be considerably difficult to expand the travel speed range of a conventional snow-removing tractor including a friction disc type transmission having a pair of friction discs arranged perpendicular to each other, without giving such unfavorable effect on the durability of the transmission.
To effectively solve such problems of conventional self-propelled working vehicle, particularly of a self-propelled working vehicle in the form of a snow-removing tractor, the present applicant has proposed in the Japanese patent application No. 58-93197, filed on May 25, 1983, a self-propelled working vehicle including an engine, a friction disc type transmission like above, and a sub-transmission of a gear changeover type interposed therebetween, the sub-transmission having a travel speed range staged into two modes. This Japanese patent application had a co-pending patent application in the Unites States, patented on May 7, 1985 as the U.S. Pat. No. 4,514,917.
In the self-propelled working vehicle according to the aforesaid Japanese patent application, the travel speed range is permitted to be favorably expanded, without giving rise to enlargement of the diametrical size of the friction disc type transmission as a main transmission and without giving mal-influences upon the durability of the main transmission.
However, even with such disposition of the sub-transmission of gear changeover type, the friction disc type main transmission with the paired friction discs perpendicular to each other is still unable to sufficiently absorb variations of the load acting thereon, thus failing to keep such load variations from reaching the sub-transmission and therebeyond to the engine, with the possibllity of affecting the durability of the sub-transmission and that of the engine.
Such problem is not so serious in such self-propelled working vehicles as according to the U.S. Patent by Mollen or the aforesaid Japanese patent application, since load variations like above can be absorbed to some extent by a friction disc type transmission which includes a pair of friction discs arranged perpendicular to each other.
However, in a self-propelled working vehicle including as a transmission thereof no more than a geared type having a travel speed range staged into, for example, five modes, those load variations experienced by the transmission are to be left as they act, without being absorbed, substantially directly on an engine.
Such inconveniency may be remarkable in an agricultural working vehicle which is intended to travel on such ground surfaces as much of variations in the undulations and ground state.
The present invention has been achieved to effectively overcome such problems of conventional self-propelled working vehicles, including the working vehicle according to the aforesaid Japanese patent application, as well as the working vehicle according to the U.S. Patent by Mollen, particularly of a self-propelled working vehicle such as an agricultural working vehicle having as a transmission thereof no more than a geared type.